Efficient conversion of benzene and syngas to toluene and xylene over ZnO-ZrO2&H-ZSM-5 bifunctional catalysts

Xiao Zhao, Xuan Shi, Zhongshun Chen, Long Xu,2, Chengyi Dai,2,*, Yazhou Zhang, Xinwen Guo,Dongyuan Yang, Xiaoxun Ma,2,*

1 School of Chemical Engineering, Northwest University, Xi’an 710069, China

2 International Science & Technology Cooperation Base for Clean Utilization of Hydrocarbon Resources, Chemical Engineering Research Center of the Ministry of Education for Advanced Use Technology of Shanbei Energy, Collaborative Innovation Center for Development of Energy and Chemical Industry in Northern Shaanxi, Northwest University, Xi’an 710069, China

3 School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

4 Shaanxi Key Laboratory of Energy Chemical Process Intensification, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China

5 Shanxi yanchang, Petroleum (Group) Corp.Ltd, Xi’an 710000, China

Keywords:ZnO-ZrO2 Bifunctional catalysts Aromatics Alkylation of benzene Syngas

ABSTRACT A series of ZnO-ZrO2 solid solutions with different Zn contents were synthesized by the urea coprecipitation method, which were coupled with H-ZSM-5 zeolite to form bifunctional catalysts.As a new benzene alkylation reagent,syngas was used instead of methanol to realize the efficient conversion of syngas and benzene into toluene and xylene.A suitable ratio of ZnO-ZrO2 led to the significant improvement in the catalytic performance,and a suitable amount of acid helped to increase the selectivity of toluene/xylene and reduce the selectivity of the by-products ethylbenzene and C9+ aromatics.The highest benzene conversion of 89.2% and toluene/xylene selectivity of 88.7% were achieved over 10%ZnO-ZrO2&H-ZSM-5(Si/Al=23)at a pressure of 3 MPa and a temperature of 450°C.In addition,the effect of the zeolite framework structure on product distribution was examined.Similar to the molecular dynamics of aromatic hydrocarbons, H-ZSM-5 zeolites comprise 10-membered-ring pores, which are beneficial to the activation of benzene; hence, the conversion of benzene is higher.H-ZSM-35 and HMOR zeolites exhibited small eight-membered-ring channels, which were not conducive to the passage of benzene; hence, the by-product ethylbenzene exhibits a higher selectivity.The distance between the active centers of the bifunctional catalysts was the main factor affecting the catalytic performance, and the powder mixing method was more conducive to the conversion of syngas and benzene.

1.Introduction

Aromatics are the most basic raw materials of the organic chemical industry,which are widely used in the synthesis of resins,fibers, rubber, dyes, and other chemical products [1-3].Aromatics mainly originate from petroleum and coal tar, and light aromatics such as benzene,toluene,and xylene(BTX)are the most important products, accounting for an important share in the petrochemical industry.Currently, about 70% of the global supply of BTX originates from naphtha cracking and catalytic reforming [4-7].However, with the increasing shortage of petroleum resources, the technology of synthesizing light aromatics from non-petroleum carbon resources has gradually become an important problem that needs to be solved.Currently, syngas has become an important resource to replace petroleum synthetic chemical products.Syngas can be obtained from widespread sources such as natural (shale)gas,coal, biomass,and carbon-containing waste,as well as carbon dioxide conversion[8-11].It is one of the important platforms for the use of non-oil resources to supply energy and chemical raw materials.

Conventionally, toluene/xylene is produced from benzene and syngas firstviathe synthesis of methanol by the selective hydrogenation of CO over a metal catalyst (such as CuZnAlOx[12],ZnZrOx[13],ZnAlOx[14],etc.),followed by the alkylation of methanol and benzene on a zeolite catalyst[15-17].Although the above two-step technical route is relatively mature, the following issues are noted:First,owing to thermodynamic constraints,the equilibrium conversion of syngas to methanol is low, and a large amount of feed gas needs to be recycled, thereby increasing operation costs.Second, the methanol self-reaction (methanol-to-olefins) is easier than the benzene and methanol alkylation reaction [18];hence, a considerable amount of methanol does not participate in alkylation, leading to the low one-way benzene conversion, and a high number of carbon deposition precursors, such as low-carbon olefins, are generated by methanol under the action of a catalyst,resulting in the facile carbon deposition on, and deactivation of,the catalyst[19,20].As a new type of a benzene alkylation reagent,syngas was used instead of methanol, and a high-efficiency benzene conversion was observed on a ‘‘metal-acid” bifunctional catalyst.The main route involves the conversion of syngas into methanol and other intermediates over a metal catalyst, followed by alkylation with benzene over a zeolite to produce toluene and xylene.Compared to the traditional methanol and benzene alkylation reaction, the one-step conversion of syngas and benzene into toluene and xylene exhibited the advantages of a high product selectivity, high benzene conversion, low by-product selectivity,and high economy.

For the direct alkylation of syngas and benzene to toluene/xylene,Olivéet al.have reported that the combination of carbonylated transition metals and AlCl3can catalyze the direct alkylation of benzene with syngas to produce alkylbenzene [21].Wanget al.have investigated the alkylation of benzene with syngas over a Zn/H-ZSM-5 zeolite byin situsolid-state nuclear magnetic resonance spectroscopy.Their group has reported that methoxy species are the reaction intermediates and that electrophilic substitution with aromatic groups occurs easily [22].Leeet al.have used a Cr2O3/ZnO&H-ZSM-5 multifunctional composite catalyst to achieve the effective conversion of syngas into xylene with a conversion of 41.4% and a xylene selectivity of 69.1% [23].Yanget al.have designed a ZnxCr/H-ZSM-5 bifunctional catalyst to realize highly selective conversion of syngas and benzene top-xylene.At a benzene conversion of 34.4%, the toluene selectivity was 73.8%, and the xylene selectivity was 20.9%, in whichp-xylene accounted for 73.2%.The bifunctional catalyst utilizes silicate-1 zeolite to coat the acid sites on the outer surface of the H-ZSM-5 zeolite, thereby improving the selectivity ofp-xylene and suppressing the formation of C9+species [24].Yuet al.have prepared Zr/H[Zn]xZSM-5 bifunctional composite catalysts with different Zn contents by impregnation and physical mixing.The total selectivity to toluene and xylene was 91%, while the benzene conversion was 18% [25].However, the low benzene conversion and low toluene/xylene selectivity are still one of the main factors that restrict the development of alkylation of benzene using syngas.

In this paper, by reasonably designing a ‘‘metal-acid” multifunctional catalyst, the bifunctional catalyst comprising ZnO-ZrO2and H-ZSM-5 were used for the alkylation of syngas with benzene to toluene/xylene.The catalyst performance was optimized by adjusting the metal composition, type of zeolite, acid amount of zeolite, and distance between the metal and zeolite.In addition,the synergistic effects of ‘‘ZnO-ZrO2&H-ZSM-5” multi-active centers in the methanol production from syngas and the alkylation of methanol and benzene were examined.The goal of a high benzene conversion (～89.2%) and a high toluene/xylene selectivity(～88.7%) was finally achieved.

2.Experimental

2.1.Catalyst preparation

ZnO-ZrO2bimetallic oxides were synthesized by the urea coprecipitation [26].Typically,Amol of Zn(NO3)2·6H2O (Aladdin,99.9%)andBmol of Zr(NO3)4·5H2O(Aladdin,99.9%)were dissolved in 140 ml of water, followed by the addition of 18 g of urea under stirring at room temperature.Then, the solution was transferred into a 250-ml flask and heated to 110°C in an oil bath.After boiling and refluxing for 8 h, the solution was precipitated overnight at room temperature, followed through centrifugation and washing several times using distilled water.The resulting product was dried overnight at 80°C and calcined at 500°C for 4 h.The obtained catalyst was denoted asxZnO-ZrO2, wherexis the molar percentage ratio of ZnO/ZnO + ZrO2(A/A+B=x A+B= 0.04 mol).In addition,ZrO2(using only 0.04 mol Zr(NO3)4·5H2O) was prepared by the same procedure.For comparison, 10% CdO-ZrO2and 10% Ga2O3-ZrO2mixed metal oxides were synthesized by the above methods,in which the total molar weight of metal ions was 0.04 mol.

Zeolites, including H-ZSM-5 with different Si/Al molar ratios(SiO2/Al2O3= 27, 36, 46, 60, 85, 120), H-ZSM-35 (SiO2/Al2O3= 25), H-MCM-22 (SiO2/Al2O3= 30), and H-MOR (SiO2/Al2O3= 25), were purchased from Nankai University Catalyst Co.Typically,H-ZSM-5 with a Si/Al ratio of 23 was used unless otherwise mentioned.

The bifunctional catalyst was prepared by mixing and grinding the two components in an agate mortar with a 1:1 mass ratio of metal oxide/zeolite unless otherwise stated.

2.2.Catalyst characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Smart Lab diffractometer using a nickel-filtered Cu Kα Xray source at a scanning rate of 0.02° over the 2θ range between 5° and 80°.The crystallite phases were identified by comparing the diffraction patterns with data of the Joint Committee on Powder Standards (JCPDS).

Scanning electron microscopy(SEM)images were recorded on a Zeiss Sigma instrument at an acceleration voltage of 5.0 kV.Some samples were sputtered with a thin film of gold.

Transmission electron microscopy(TEM)images were recorded on a Tecnai G2 20 S-twin instrument(FEI Co.)with an acceleration voltage of 200 kV.TEM samples were ultrasonicated in ethanol,dropped onto carbon-coated copper grids, and dried under ambient conditions.High-resolution TEM (HRTEM) images were recorded to assign the d-spacing of ZnO-ZrO2for the comprehensive understanding of the crystal structure.Moreover, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) mapping images of the catalysts were recorded at 200 kV.

Temperature-programmed desorption of NH3(NH3-TPD, ASAP 2920, Micromeritics, US) was employed to characterize the acid properties of the catalyst, and the signal was detected by mass spectrometry.The sample was heated to 500 °C at a rate of 10 °C·min-1, held at this temperature for 60 min, and then cooled to 50 °C to perform ammonia gas adsorption at a heating rate of 10 °C·min-1to 600 °C for temperature-programmed desorption,and the desorbed NH3was monitored using a thermal conductivity detector.ChemMaster was employed to calculate the amount of acid.

2.3.Catalytic tests

Alkylation of syngas and benzene was employed in a fixed-bed reactor with a continuous flow at high pressure.Typically,the catalyst (1.0 g) with grain sizes of (0.42-0.85 mm) was loaded in a quartz reaction tube(inner diameter,8 mm)to investigate the catalytic performance using syngas with a H2/CO ratio of 2:1 and a gas hourly space velocity (GHSV) of 7200 ml·g-1·h-1.Benzene was pumped into the fixed-bed reactor by using an advection pump,with a liquid hourly space velocity (LHSV) of 1.8 h-1.There was no requirement for the catalyst to be reduced,and the reaction was conducted at 450 °C and 3 MPa.The products were gasified at 200 °C and analyzed on-line by a gas chromatograph (GC-6890A)with a KB-Wax capillary column (30 m × 0.32 mm) using an FID detector.Benzene conversion and product selectivity were calculated as follows:

Benzene conversion:

AB,AEB,AT,AX,represent the peak areas of benzene, ethylbenzene, toluene, xylene andaromatics on the gas chromatography,andFB,FEB,FT,FX,FC+9represent the molar correction factors of benzene, ethylbenzene, toluene, xylene andaromatics on the gas chromatography, respectively.

The catalytic activities were expressed as moles of benzene converted to T, X, EB andaromatics per gram of catalyst per hour(space time yield, STY).

3.Results and Discussion

3.1.Structural characterization of catalysts

Fig.1 shows the XRD patterns of MxOy-ZrO2(M = Zn, Cd, Ga)bimetallic oxides obtained by the urea co-precipitation method.Broad peaks were observed at 30.3°, 35.2°, 50.4°, 60.2°, 63.0°, and 74.5°, corresponding to the characteristic diffraction peaks of tetragonal ZrO2(t-ZrO2) (JCPDS NO.88-1007).After the addition of ZnO (1%-33%) to ZrO2, diffraction peaks related to ZnO were not observed in the XRD spectrum, and the crystal phase was still t-ZrO2(Fig.1(a)).Similar to ZnO-ZrO2, the addition of CdO and Ga2O3to ZrO2did not change the crystal phase(Fig.1(b)),indicating that MxOy-ZrO2(M = Zn, Cd, Ga) forms a solid solution phase,where M is doped into ZrO2in the lattice matrix [27,28].Fig.2 shows the microscopic analysis of 10% ZnO-ZrO2.HRTEM revealed that the crystal plane spacing of 10%ZnO-ZrO2is ～0.299 nm(Fig.2(a)), corresponding to the (1 0 1) crystal face of tetragonal-phase ZrO2, which is consistent with the XRD results.At the same time,the element distribution analysis in Fig.2(b) revealed that Zn is highly dispersed in ZrO2, further confirming the formation of a solid solution phase.

3.2.Effect of metal composition on the alkylation of syngas and benzene

Effects of 10%MxOy-ZrO2(M=Zn,Cd,Ga)and H-ZSM-5(23)on the alkylation of syngas with benzene were investigated under the conditions of 450°C,3 MPa,GHSV=7200 ml·g-1·h-1,LHSV=1.8 h-1.As can be observed from Table 1,10%of the MxOy-ZrO2(M=Zn,Cd,Ga) solid solution catalyst coupled with H-ZSM-5 (23) exhibited high selectivity for the alkylation of syngas and benzene, but the benzene conversion was quite different.The highest selectivity of toluene/xylene over CdO-ZrO2&H-ZSM-5 was 92.7%, but the benzene conversion was only 44.9%.On the other hand,a high benzene conversion was observed over ZnO-ZrO2&H-ZSM-5 while maintaining high selectivity.In order to evaluate the utilization of CO,CO efficiency is introduced, as shown in Fig.S1.This value is defined as the proportion of C atoms which are converted into methyl and other chemical groups of aromatics products in all converted CO[24].It can be found that the gaseous products of MxOy-ZrO2&H-ZSM-5 the three are mainly composed of CO2and lowcarbon hydrocarbons.The highest conversion of CO over Ga2O3-ZrO2&H-ZSM-5, but CO efficiency was low.The CO efficiency of ZnO-ZrO2&H-ZSM-5 was the highest, so the selectivity of xylene was higher.

The effect of the ZnO-ZrO2ratio on the alkylation of syngas with benzene is further investigated.A high toluene/xylene selectivity,albeit a low benzene conversion, was observed over ZrO2&HZSM-5 (Fig.3 and Table S1).The addition of a small amount of ZnO into ZrO2led to the significant improvement in the benzene conversion as well as a high toluene/xylene selectivity.However,at a ZnO-ZrO2ratio of greater than 10%, the benzene conversion decreased.At the same time, with the increase in the ZnO-ZrO2ratio from 0 to 10%, the toluene selectivity gradually decreased,while the xylene selectivity increased.With the continuous increase in the ZnO content, the contents of ethylbenzene and C9+aromatics increased.Fig.S2 shows the conversion of CO and gaseous product distribution of alkylation of benzene with syngas overxZnO-ZrO2&H-ZSM-5.The selectivity of C2-C40increases with the increase of ZnO content,while the CO efficiency decreases with the increase of ZnO content.This may be due to lead to a too high surface density of H species when ZnO is excessive,resulting in the formation of a large number of methanol intermediates.The intermediates reacted to form olefins over H-ZSM-5 and then hydrogenated rapidly; the self-reaction of methanol also inhibited the alkylation of benzene.Therefore, the optimum ZnO-ZrO2ratio for the alkylation of syngas and benzene to toluene/xylene is 10%,and the selectivity of toluene/xylene is up to 88.7% without sacrificing the benzene conversion.As ZrO2exhibited a weak ability to activate H2, it exhibited a low methanol synthesis activity.The results revealed that ZrO2can activate CO through its oxygen vacancy to form formate species, followed by hydrogenation to methanol in the presence of H2[29-32].If a component can activate H2, it can accelerate the formation of methanol [32,33].ZnO is well known to be conducive to the adsorption and activation of H2[34,35]and the zinc species on ZnO-ZrO2nanoparticles can accelerate the activation of H2,thus providing an increased number of H species for the hydrogenation of chemically adsorbed CO species,producing an increased amount of methanol[31],providing a sufficient amount of the alkylation reagent for benzene, and improving the benzene conversion.However, excess zinc species can accelerate the formation rate of methanol intermediates,resulting in the deep alkylation reaction to produce heavier aro-matics; moreover, methanol self-reaction (such as MTO) is very easy to carry out, and the olefins generated by MTO reaction are easy to react with benzene to form ethylbenzene (Table S1).On the catalyst composed of ZnO-ZrO2and H-ZSM-5 with high ZnO content,the selectivity of C2-C4=in the gaseous products is higher,which further proves the above conclusion(Fig.S2).Therefore,it is imperative to control the hydrogenation ability to inhibit byproducts and obtain high toluene/xylene selectivity.

Fig.1. XRD patterns of metal oxides prepared by urea co-precipitation.(a) x ZnO-ZrO2 and (b) 10% MxOy-ZrO2 (M = Zn, Cd, Ga).

Fig.2. Microscopic analysis of 10% ZnO-ZrO2.(a) HRTEM image and (b) STEM-EDS elemental mapping.

Furthermore, a 10% ZnO-ZrO2catalyst with the best performance for the alkylation of syngas and benzene was selected to investigate the effect of temperature on the reaction results.Temperature considerably affected the benzene conversion, and the benzene conversion increased from 41.4% to 89.2% from 400 to 450 °C (Fig.4).With the increase in the reaction temperature,the toluene selectivity decreased, while the xylene selectivity increased.The increase in temperature is not conducive to syngas to methanol,which reduces the CO efficiency,but the conversion of CO increases with the increase in temperature(Fig.S3).This result is probably related to the fact that with the increase in temperature, the speed of alkylation is accelerated, and the rapid reaction of benzene with methanol also drives the speed of methanol synthesis, thus increasing the benzene conversion.

3.3.Effect of acidity of H-ZSM-5 zeolite on reaction results

ZSM-5-type zeolites offer a strong and tunable acidity,excellent shape selectivity and good hydrothermal stability, leading to their widely use in petrochemical fields [36-38].In addition, it is an active catalyst for the alkylation of methanol and benzene.Therefore,the typical benzene and methanol alkylation catalyst ZSM-5 is first selected and then combined with double metal oxide ZnOZrO2as a bifunctional catalyst for the alkylation of syngas with benzene.Fig.S4 shows the SEM images of H-ZSM-5 with different Si/Al ratios.It can be seen that the crystallinity of zeolite is high,the particle distribution is uniform, and the average particle size is about 1-2 μm.Fig.5(a)shows the XRD patterns of H-ZSM-5 with different Si/Al ratios.Diffraction peaks were observed at 7.9°, 8.8°,23.1°, 23.9°, and 24.3° for all samples, exhibiting a typical MFI topology.The NH3-TPD profiles in Fig.5(b) revealed that with the increase in the Si/Al ratios, the acid strength decreases.Fig.S5 and Table S2 show the quantitative analysis of NH3-TPD.With the increase of Si/Al ratio, the acid content and acid strength decrease.The sharp decrease in the peak area indicated that the density of acid sites significantly decreases.The density of acid sites will affect the distribution of aromatic products.With thedecrease in the acid site density, the selectivity of C9+aromatics andxylene sharply increased,but it gradually stabilized at a certain acid site density, while with the increase in the acid site density,the selectivity of toluene clearly increased(Fig.5(c)).Hence,xylene possibly originates from the alkylation of toluene,while C9+aromatics possibly originate from the deep alkylation of toluene and xylene.In the Py-FTIR spectra shown in Fig.S6, the peaks at 1547 and 1447 cm-1are attributed to pyridine adsorbed on Br?nsted acid and Lewis acid sites, respectively.The alkylation of benzene with methanol is an electrophilic alkylation reaction catalyzed by Br?nsted acid.It can be seen from the figure that the appropriate amount of Br?nsted acid is conducive to the alkylation of benzene with syngas.From the results shown in Fig.5(d) and Table S3, the appropriate Si/Al ratio can simultaneously improve the benzene conversion and toluene/xylene selectivity.Fig.S7 shows the gaseous products data of the reaction.From the activity point of view,the conversion of CO is positively correlated with the conversion of benzene.The selectivity of carbon dioxide is about 50%;The gaseous products are mainly methane and C2-C4alkanes,and the remaining is a small amount of C2-C4olefin and C5+hydrocarbons.The increase of Si/Al ratio makes the selectivity of heavy aromatics in aromatics products, so the CO efficiency is improved.

Fig.4. Effects of temperature on reaction performance.Reaction conditions: 400-450 °C, 3 MPa, H2/CO = 2/1, GHSV = 7200 ml·g-1·h-1, LHSV = 1.8 h-1.

3.4.Effect of zeolite framework structure on alkylation of syngas and benzene

The effect of zeolite framework structure on the alkylation of syngas with benzene was investigated.The product distribution considerably depended on the framework structure of the zeolites used in the bifunctional catalyst (Table 2).A high toluene/xylene selectivity was easily obtained over ZnO-ZrO2&H-ZSM-5 and ZnO-ZrO2&H-MCM-22 catalysts, while a higher ethylbenzene selectivity was observed over ZnO-ZrO2&H-ZSM-35 and ZnOZrO2&H-MOR catalysts.This result may be attributed to the following reasons:H-ZSM-5 and H-MCM-22 zeolites exhibit large micropores, such as 10-membered-ring or 12-membered-ring channels;hence, these catalysts exhibit a high toluene/xylene selectivity in the alkylation of syngas with benzene.H-ZSM-35 and H-MOR zeolites with 8-membered-ring channels can easily adsorb intermediates such as ketene and promote the formation of ethylene[39,40],leading to the increase in the by-product ethylbenzene.This view was also confirmed by the higher selectivity of C2-C4=in the gaseous products.Due to the limitation of the channel size, the alkylation of benzene was inhibited.Therefore, the conversion of benzene and the CO efficiency were both low (Table S4).H-ZSM-5 with 10-membered-ring channels exhibited the highest activity for alkylation of syngas with benzene due to the similar kinetic diameters of aromatic molecules, with a benzene conversion of 89.2%.A low benzene conversion was observed over ZSM-35 and H-MCM-22 due to their small pore size, which was not conducive to the diffusion of benzene.

Table 2Effect of zeolite framework structure on the alkylation of syngas and benzene over 10% ZnO-ZrO2&Zeolite bifunctional catalyst

3.5.Effect of distance of bifunctional catalysts on catalytic performance

Fig.5. Selective conversion of syngas and benzene to toluene/xylene over 10% ZnO-ZrO2&H-ZSM-5.(a) XRD patterns and(b)NH3-TPD profiles of H-ZSM-5 at different Si/Al ratios; (c) effect of acid site density on product selectivity; (d) catalytic performance as a function of Si/Al ratios.Reaction conditions: 450 °C, 3 MPa, H2/CO = 2/1,GHSV = 7200 ml·g-1·h-1, LHSV = 1.8 h- 1.

By analyzing the relationship between the temperature and Gibbs free energy for benzene alkylation and methanol synthesis,methanol synthesis at 450°C was thermodynamically unfavorable,while the alkylation of methanol with benzene was thermodynamically favorable at a higher temperature(Fig.6).Therefore,methanol synthesis from syngas and coupling of methanol with benzene alkylation are designed.Methanol synthesis is driven by the alkylation of methanol with benzene,thus promoting the reaction.The Gibbs free energy ΔrG= -0.6 kJ·mol-1at 450 °C indicated that a one-step method is thermodynamically feasible.The distance between the two active centers affected the catalytic performance of bifunctional catalysts[41,42].In this study,the effect of the distance between ZnO-ZrO2and H-ZSM-5 components on the alkylation of syngas with benzene was investigated.Fig.7I catalyst was packed in the double-bed mode, and the two components were separated by quartz cotton,which not only reduced the selectivity of xylene but also considerably reduced the benzene conversion;these results are probably related to the disadvantage of methanol formation at 450 °C.By changing the mixing method of the two active components, from the dual-bed mode to the 0.42-0.85 mm granule-mixing mode (Fig.7II) and to the powder-mixing mode (Fig.7III), further reducing the distance between the two active centers.With the decrease in the distance between ZnOZrO2and H-ZSM-5,the benzene conversion significantly increased(Fig.7).By comparing the conversion of CO under different compounding methods, it can also be found that the conversion and efficiency of CO increase with the decrease of the distance between the two active centers (Table S5).The shorter the distance, the more rapid the alkylation of the methanol formed on ZnO-ZrO2particles with benzene in H-ZSM-5,thereby producing an effective thermodynamic driving force and promoting CO to produce an increased number of methanol intermediates as alkylation reagents;hence,the benzene conversion is considerably improved.In addition, the experimental results of using 10% ZnO-ZrO2&HZSM-5 catalysts with different particle sizes show that the reaction is not affected by internal diffusion (Table S6).Increasing the gas space velocity of the catalyst,the conversion of benzene remained almost unchanged,indicating that the catalyst was not affected by external diffusion (Table S7).

Fig.6. Gibbs free-energy changes for benzene alkylation and methanol synthesis.

Fig.7. Effect of distance between 10% ZnO-ZrO2 and H-ZSM-5 on catalytic performance.Reaction conditions: 400-450 °C, 3 MPa, H2/CO = 2:1,GHSV = 7200 ml·g-1·h-1, LHSV = 1.8 h-1.

4.Conclusions

In summary,the effect of the MxOy-ZrO2solid solution(M=Zn,Cd, Ga) bimetallic on the reaction performance was investigated.Results revealed that the best performance is observed by using Zn as the promoter for hydrogen activation.At the same time, by adjusting the doping amount of Zn, in the alkylation reaction of syngas and benzene, the highest activity was observed with 10%ZnO-ZrO2,and the benzene conversion was up to 89.2%when coupled with H-ZSM-5.The appropriate amount of an acid can improve the selectivity of toluene/xylene and reduce the selectivity of ethylbenzene and C9+aromatics.At a Si/Al ratio of 23, the toluene/xylene selectivity was 88.7%.The zeolite framework structure significantly affected the product distribution of alkylation of syngas and benzene.H-ZSM-35 and H-MOR with 8-memberedring channels exhibited a higher selectivity for ethylbenzene.On the other hand, H-ZSM-5 zeolite with 10-membered-ring pores,similar to the molecular dynamics diameter of aromatics,exhibited the highest conversion of benzene and a better product selectivity.The distance between the two active components of ZnO-ZrO2and H-ZSM-5 significantly affected the catalytic activity.The dual-bed filling mode led an extremely low conversion of benzene, but the benzene conversion was considerably increased by the powdermixing mode, indicating that the synergistic effect of the bifunctional catalyst promotes the reaction.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the financial support from the National Key Research & Development Program of China (2018YFB0604901),the National Natural Science Foundation of China (21706210)and the Key Research&Development Program of Shaanxi Province(2020ZDLGY11-06).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.05.028.